Automatic injection devices offer an alternative to manually-operated syringes for delivering substances into patients' bodies and allow patients to self-administer injections. Automatic injection devices have been used to deliver medications under emergency conditions, for example, to administer epinephrine to counteract the effects of a severe allergic reaction. Automatic injection devices have also been described for use in administering anti-arrhythmic medications and selective thrombolytic agents during a heart attack (see e.g., U.S. Pat. Nos. 3,910,260; 4,004,577; 4,689,042; 4,755,169 and 4,795,433). Various types of automatic injection devices are also described in, for example, U.S. Pat. Nos. 3,941,130; 4,261,358; 5,085,642; 5,092,843; 5,102,393; 5,267,963; 6,149,626; 6,270,479; and 6,371,939 and U.S. Patent Publication No. WO/2008/005315.
Conventionally, an automatic injection device, when operated, causes a syringe in the device to move forwardly and a needle to project from a housing so that a substance contained in the syringe is ejected into a patient's body. In some cases, movement of the syringe toward the patient's skin such that the needle is inserted into the skin before pressurizing a substance inside the syringe helps prevent the substance from dripping out of the needle before the injection occurs.
Conventional automatic injection devices can occasionally fail due to suboptimal minimum forces (FtFs) required to actuate their firing mechanisms. Conventional devices can misfire even when their firing mechanisms are not engaged with a substantial amount of force, or fail to fire even when their firing mechanisms are engaged with a substantial amount of force. For example, in a conventional device with a lower than optimal FtF, an inadvertent tap on the firing mechanism may engage the firing mechanism and cause the device to misfire and expel the substance contained in the device. This may lead to wastage or misdelivery of the substance before the patient has attached the automatic injection device to his/her body for an injection. Conversely, in a conventional device with a higher than optimal FtF, even a moderate or large force applied by a patient on the firing mechanism may fail to engage the firing mechanism and may fail to expel the substance contained in the device. This may require patients to apply excessive amounts of force to the firing mechanism to expel the substance, which may be uncomfortable to many patients and even intolerably uncomfortable to particularly frail patients. Such variability in the FtF required to actuate the firing mechanism is not desirable in automatic injection devices.